Trump said to a crowd in July 2016,

I think I have the best temperament or certainly one of the best temperaments of anybody that’s ever run for the office of president. Ever. Because I have a winning temperament. I know how to win.

He made similar claims at other campaign rallies and during the debates with Hillary Clinton.

ABC News – Donald Trump Says He Has One of the ‘Best Temperaments’ of Any Presidential Candidates
US News & World Report – Temperament Tantrum